This invention relates to shaping tools, and more particularly to concave reamers used on small bones to make convex shapes on small bone ends in procedures for fusing joints.
Making a convex surface through the use of a tool is generally disclosed in prior art. U.S. Pat. Nos. 1,927,463 to Mcintosh, 4,798,503 to Huju, 4,997,322 to Wells, and 5,004,383 to Elliot disclose cutting tools that produce convex surfaces. U.S. Pat. No. 4,997,322 discloses a reamer for removing spot welds during automotive body repairs. U.S. Pat. No. 4,798,503 discloses a tool for forming tenons (conical shapes) onto the end of wood posts that is used with a hand drill.
The prior art includes tools used for shaping convex surfaces onto bone. For example British Patent No. 1,277,769 to Charnley discloses two tools, a concave and a convex, for use in hip joint surgery. The concave tool is a solid concave structure with cutting teeth along the inner surface for cutting the bone surface. U.S. Pat. No. 4,246,895 to Rehder discloses an apparatus for producing a conical, outwardly tapering surface on a bone, specifically the femur head of the hip joint.
French Patent Nos. 2,304,322 and 2,554,709 disclose tools for making convex surfaces on bone. German Patent Nos. DT 2,834,297 and DE 2,748,452 show milling cutters that create a hemispherical contour on a femoral bone prior to insertion in an acetabular prosthesis.
The prior art concave reamers for use on bones have been found deficient for use in fusing the joints of small bones in particular. The prior art reamers generally have a concave shape containing cutting blades along the inner surface. During operation, such reamers trap heat due to the contact with the bone surface and allow the accumulation of bone chips between the work surface and the reamer.
Some of the prior art reamers have attempted to solve this problem by having holes or slots within the concave head of the reamer to allow bone chips to be cleared and air to circulate to cool the bone surface. This design has proved inadequate, however, because bone chips can clog the holes of the reamer, and cause the bone surface to overheat. Bone is a living substance, which if overheated will die, preventing the healing process necessary for a successful fusing operation. The possible necrosis of the bone is especially detrimental in an operation to fuse joints, because the tissue must remain live to heal properly.
Another disadvantage of many of the prior art tools is that they form conical surfaces through the use of straight blades, such as U.S. Pat. No. 4,798,503 to Huju. It is, however, desirable to provide the bone with a spherical surface for the best results in a joint fusing operation.
Also, prior art tools that are designed to form partial spherical surfaces are often complicated to manufacture, and therefore expensive. For example, English Pat. No. 1,277,769 requires that cutting teeth be provided over a substantial part of the inner curved surface of the concave tool. This piece would be difficult and expensive to manufacture, especially in sizes small enough for use on small bones in the hands and feet.